About reversed phase interter to the power tubes

[pjd3]

Hello,

I was hoping for a sanity check or deepening of the understanding of what happens when the outputs of the phase inverter are not wired correctly.

Does the problem have to do with the phase of the negative feedback causing a positive addition rather than a negative cancellation? It was one of those things while analyzing the schematic last night where I would see the issue then my brain would lose it. Hate when that happens.

But yeah, my first 2 builds were done from scratch which actually made things much more concise and clearer when building. This 3rd amp project is a rebuild to another type of Fender so, its a matter of working through components and wiring that are already there making it harder to see things and focus on one thing at a time.

Thank you for helping me to clarify this little brain teaser. I've heard that you get some really bad sonic artifacts should you switch the wiring on the phase inverter!

Thank you!
Best,
Phil D

[martin manning]

...what happens when the outputs of the phase inverter are not wired correctly.
Does the problem have to do with the phase of the negative feedback causing a positive addition rather than a negative cancellation?

Correct. Doing that (or reversing the OT primary leads) flips the output signal 180 degrees, et voilà, positive feedback, and you have an oscillator.

[pjd3]

Hi Martin and thank you.

So, this probably tells me that when referring to the layouts of these amps, it is imperative to pay attention to which output tube the brown and blue wire go to as they will absolutely correlate to the specific phase inverter output wire.

What is happening at the phase inverter outputs has always made sense and has been clear, but it has been what is going on with the OT that has alluded me. It was never apparent just looking at the wires as they protrude from the OT - only the color of the wires have been what tells me where they go. OK. That's great. think I've got it now and can confidently wire up the PI/OT/power tube section without creating sonic hell.

Thanks so much Martin,
Best,
Phil D

[LOUDthud]

I never really understood feedback until I started working with opamps. When I was learning tube theory they always talked about feedback as being "out of phase". Truth is that in any non-inverting amp like a guitar power amp, the feedback is actually in phase with the input (and naturally the output). It's the differential amp (the PI) that does the phase inversion so that the feedback subtracts from the input signal. If you draw little arrows on a schematic to indicate phase, you see that the top power tube's plate is in phase with the input of the PI and the top side of the secondary also is in phase when the bottom of the secondary is grounded.

[pjd3]

Thank you all for chiming in on this, -definitely helped to sort things out, and give a few things to think about and inspect in the circuit.

Pretty much any OT I've ever had, I've done the ac voltage ratio/winding ratio/impedance ratio/tap determination just to see for myself and give exercise to the reflective properties of an OT. Next time, I will very likely make it a point to look at the phase of the test ac voltage to the phase of the tap(s) phase, just to get even more intimate with the orientation of the typical OT and its wire color/tap color designations vs phase.

best,

Phil D.

[martin manning]

Next time, I will very likely make it a point to look at the phase of the test ac voltage to the phase of the tap(s) phase, just to get even more intimate with the orientation of the typical OT and its wire color/tap color designations vs phase.

Easy. See here: https://ampgarage.com/forum/viewtopic.p ... 65#p460065